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Abstract

An integrated sensor system is presented which displays passive long range operation to 100 km at pico-strain (pε) sensitivity to low frequencies (4 Hz) in wavelength division multiplexed operation with negligible cross-talk (better than −75 dB). This has been achieved by pre-stabilizing and multiplexing all interrogation lasers for the sensor array to a single optical frequency reference. This single frequency reference allows each laser to be locked to an arbitrary wavelength and independently tuned, while maintaining suppression of laser frequency noise. With appropriate packaging, such a multiplexed strain sensing system can form the core of a low frequency accelerometer or hydrophone array.

Figures (5)

The reflection spectra of three closely spaced FFPI channels. Clearly shown are two Fabry Perot modes, which are impedance matched. Only one mode is employed for sensing in each channel. The out-of-band sidelobes, due to imperfect apodization of the FBGs, shown here are not to scale. Coupled with reflections from the slight impedance mismatch, these can cause inter-channel cross-talk.

Simplified overview schematic of the sensor system. Upper circuit: A number of lasers are multiplexed to a single frequency reference, which performs a laser frequency to RF phase conversion. Subsequent demultiplexing, and low pass filtering to remove residual FRF, enables noise suppression of each individual laser via a phase measurement and a feedback via a servo to the laser current. Lower circuit: Each laser is locked to a Fabry Perot mode of each sensor, with feedback to the laser current and sensor readout achieved via an RF modulation technique. Conversion of PM to AM at the FFPI creates a measure of the sensor strain.

An out-of-loop measurement yields A) the frequency noise of the laser when locked to the frequency reference compared to B) the frequency noise of the free running laser. The small resonances at 20 Hz and 480 Hz are not laser frequency noise but an artifact introduced by mechanical pick-up

A) Strain spectra of each of the four sensors interrogated by its respective pre-stabilized laser, with all lasers operating concurrently. B) The strain spectra of one sensor when a free running laser is used to interrogate it. The suppression at 10 Hz of frequency noise is approximately two orders of magnitude.

Strain spectrum of the lowest noise channel A) with only a few meters of fiber delivering the light to and from the sensors and B) with 100 km of delivery fiber. The slight rise in the noise floor is negligible and is due to the lower signal to electronic noise, when the light arrives at the photo-detector after 100 km of fiber and net signal strength decline of −10 dB.